Borehole stabilisation

ABSTRACT

A method for stabilising an unconsolidated zone ( 18 ) of a borehole, by forming a region of the borehole having enlarged diameter ( 22 ) in the unconsolidated zone; positioning a fluid-filled casing ( 26 ) in the borehole the liner having a pipe ( 28 ) extending therethrough into a lower portion; pumping cement from the surface inside the pipe so as to exit the liner at the lower portion and flow upwards to fill the annulus ( 44 ) formed between the outside of the liner and the borehole in the unconsolidated zone; withdrawing the pipe ( 28 ) from the casing while pumping fluid therethrough so as to maintain the liner substantially fluid-filled and to displace cement above the liner in the adjacent region; drilling through the cement and liner in the unconsolidated zone after the cement has set.

[0001] The present invention relates to borehole stabilisation and isparticularly applicable to methods for stabilising boreholes involvingthe use of drillable liners or casings.

[0002] The cementing of casings in boreholes for stabilisation and zonalisolation is well known, especially in the field of well construction inthe oil and gas industry. Such operations are performed at variousstages while the well is being drilled. In a casing operation, the drillstring used to drill the borehole is withdrawn and a tubular casing runinto the borehole such that a space is left between the outer surface ofthe casing and the wall of the borehole (an annulus). Cement slurry isthen pumped down the inside of the casing such that it exits the casingat the bottom and flow back towards the surface in the annulus. When thecement sets, the casing is fixed in the borehole and the various layersthrough which the borehole passes are supported and isolated by thecement and casing. Drilling is then resumed, but at a smaller diameterthan previously because of the presence of the casing.

[0003] Setting casing can be useful to support a weak formation or toseal off zones of high or low pressure to prevent uncontrolled influx offluids or damage to the formation due to the use of dense drillingfluids to balance high formation pressures. For deep wells, it can benecessary to set casing several times before the borehole is drilled tothe target depth. Since each casing reduces the diameter of the boreholebelow that point, planning of the well can be difficult to stay withinestablished drilling practices or reasonable cost while still obtaininga well of useful size. In certain cases, as many as ten casingoperations have been necessary.

[0004] One reason for setting casing is to support a weakly consolidatedformation that is becoming worn away as drilling proceeds or that can befractured. In such cases, the level of reinforcement required can beobtained from a relatively thin layer of cement alone without the needfor casing to be present, especially where strong cements such asfibre-reinforced cements are used. It has been previously proposed tostabilise such zones by enlarging the borehole in that zone byunder-reaming and setting a short section of casing across the zone andcementing it in. After the cement has set, the casing and some of thecement is drilled out at the same diameter as was being used before thecementing operation, leaving the under-reamed section with a layer ofcement on the borehole wall. Such an operation is described in U.S. Pat.No. 5,842,518, and other operations involving drillable casing aredescribed in U.S. Pat. No. 5,957,225.

[0005] Another approach to the problem of reduction of borehole diameterwhen installing several casings is described in U.S. Pat. No. 6,098,710and U.S. Pat. No. 6,267,181. In this case a special casing tool is usedto avoid the need for an annulus in the zone above by diverting fluidflow back into the borehole. Since this allows the use of casing ofsubstantially the same diameter as that above, it is not necessary todrill out the casing when continuing with the drilling operation.

[0006] It is an object of the invention to provide methods and apparatuswhich allow weak zones of boreholes to be supported without excessivelyreducing the diameter of the borehole at that point.

[0007] One aspect of the present invention provides a method ofstabilising a zone of interest of a borehole, comprising:

[0008] (i) forming a region of the borehole having enlarged diameter inthe zone of interest when compared to the adjacent zone above;

[0009] (ii) positioning a liner in the borehole extending across thezone of interest and into the adjacent zone above, the liner having apipe extending therethrough to a lower portion and being connected to acement supply at the surface;

[0010] (iii) pumping cement from the surface inside the pipe so as toexit the liner at the lower portion and flow upwards to fill the annulusformed between the outside of the liner and the borehole in the zone ofinterest and to extend into the adjacent zone above;

[0011] (iv) withdrawing the pipe from the liner; and

[0012] (v) drilling through the cement and liner in the zone of interestafter the cement has set to form a borehole of substantially similardiameter to that of the adjacent zone above.

[0013] The portion of the liner extending into the adjacent zone abovepreferably has a smaller diameter than that part of the liner in thezone of interest. By reducing the diameter of the liner at its upperpart, it is possible to avoid a flow restriction in the annulus at thepoint where the liner enters the zone above. Alternatively, the linercan be of substantially constant diameter with sufficient clearance inthe upper zone to allow proper cement placement. By extending the cementsheath in the zone above, the sheath is anchored in the upper zoneleading to better stability. The term “liner” used here refers equallyto casings.

[0014] It is preferred that the liner be in a fluid filled state whenplaced in the zone, and that fluid be pumped through the pipe as it iswithdrawn from the liner so as to maintain the fluid-filled state and todisplace any cement above the liner in the zone above. The fluid used tofill the liner can typically be a heavy mud or other fluid so as toprevent buoyancy of the liner when installed in the well.

[0015] The zone above the zone of interest can be an open,well-consolidated and stable zone, of can be lined with cemented casingor sheath. Alternatively, it can be a zone that has been previouslystabilised by use of a drilled liner, either according to the presentinvention or by another method. The region of enlarged diameter can beformed using an under-reamer or a bi-centre bit. It is preferred that abit pilot hole is provided at the bottom of the enlarged section.

[0016] The pipe in the liner is preferably drill pipe and the liner isconnected to the drill pipe by means of a setting tool. This can connectto the upper part of the liner by means of a threaded connector or anyother releasable connector. Centralisers can be provided to centralisethe liner, both in the zone of interest and in the zone above. A dartlanding sub can be provided inside the drill pipe near the bottomthereof.

[0017] The drill pipe also can be connected to a float shoe at thebottom of the liner which has side ports for communication with theannulus and cross webs to engage in the pilot hole. Centralisers can beprovided for centralising the drill pipe in the liner.

[0018] When the cement has been placed, the drill pipe is disconnectedfrom the liner and displacing fluid circulated through the drill pipeinside the liner to prevent cement from filling the liner and todisplace cement above the liner.

[0019] The cement used is preferably a fibre-reinforced cement. Thefibres can be metallic or formed from a suitable polymeric material. Lowdensity, non-foamed slurries are preferred.

[0020] The steps described above can also be preceded by installing acasing in the zone above which has a drillable oversized casing shoeinto which the liner is installed and cemented.

[0021] The methods of the present invention can be used for stabilising“problem zones” of boreholes in different situations, for example: informations with mechanical stability problems, formations experiencingshear failure or plastic flow, unconsolidated formations, formationswith narrow pore or fracture gradient margins or narrow kick or losswindows, and formations susceptible to differential sticking problemsdue to large differences between drilling fluid hydrostatic pressure andformation pressure. In particular, the method has application to zoneswhich need to be drilled either with a mud weight lower than thestability window, or higher than the fracture gradient, in order to savea casing point, or when there is no safe mud weight.

[0022] The invention will now be described by way if example, withreference to the accompanying drawings, in which:

[0023]FIG. 1 shows a portion of a borehole extending through an problemzone;

[0024]FIG. 2 shows the installation of drillable liner into the problemzone;

[0025]FIG. 3 shows cement being pumped into the annulus of the problemzone;

[0026]FIG. 4 shows a dart landing in the drill pipe at the end ofcementing;

[0027]FIG. 5 shows the drill pipe as it is withdrawn from the liner;

[0028]FIG. 6 shows the liner cemented in place;

[0029]FIG. 7 shows the liner partially drilled out; and

[0030]FIG. 8 shows the borehole after the liner has been drilled out.

[0031]FIG. 1 shows a partial example of a borehole to which theinvention typically applies. The borehole 10 has been drilled from thesurface and at least one casing 12 has been cemented 14 to stabilise andisolate the zones penetrated 16. Further drilling has caused theborehole to enter a relatively thin problem zone 18. This formation canbe one that has mechanical stability problems, a formation experiencingshear failure or creep (plastic flow), an unconsolidated formation(salt, coal, shale, etc.), a formation with narrow pore or fracturegradient margins or narrow kick or loss windows, or a formationsusceptible to differential sticking problems due to large differencesbetween drilling fluid hydrostatic pressure and formation pressure. Insuch cases, there can be great danger of drilling problems such assticking, fluid loss or influx (kick), even though the zones below 20might be stable. In order to prepare the borehole for stabilisationaccording to the invention, the diameter of the borehole in the problemzone 18 has been enlarged 22 using an under-reamer or bi-centre bit (notshown). A bit pilot hole 24 is formed at the bottom of the enlarged zone22.

[0032] Referring now to FIG. 2, a fluid-filled, drillable liner 26 isrun into the borehole 10 on drill pipe 28, so as to extend through theenlarged zone 22. The liner 26 is supported on the drill pipe 28 bymeans of a liner setting tool 30 at the top and a float shoe 32 at thebottom. The liner setting tool 30 has a threaded portion 34 whichengages corresponding threads 36 in the liner 26. The lower part of thedrill pipe 28 includes a dart landing sub 29 and connects to the floatshoe 32 by means of a stab-in receptacle 38. Centralisers 27 areprovided on the drill pipe 28 to allow centralisation in the liner 26and to facilitate connection with the float shoe 32. The float shoe 32also includes a self-filling float valve 40 and side ports 42 thatprovide a fluid connection between the drill pipe 28 and the annulus 44outside the liner 10. The float shoe 32 also includes cross webs 46 atits lower end which engage the formation at the bottom of the pilot hole24 when the liner is set on bottom.

[0033] The main part 26′ of the liner 26 has a diameter that ismarginally smaller than that of the casing 12 above the enlarged zone22. The portion 26′″ of the liner 26 extending into the casing 12 abovehas a smaller diameter in order that sufficient annular space for fluidflow is obtained and the portion 26″ of the liner 26 joining the upper26′″ and lower 26′ parts has a progressive diameter change (shell head)in order that there is no flow restriction between the annulus in theenlarged region 22 and the annulus in the casing 12 above.

[0034] The liner 26 is centralised in the enlarged zone 22 by bow springcentralisers 48 and in the upper casing 12 by smaller blade centralisers50. The centralisers 48, 50 can be made of metal, metal composites orother fibre reinforced materials. Apart from their centralising role, itis important that both of these should be drillable, if possible in amanner that will not affect any cement in which they may be embedded.While blade and bow spring centralisers are shown here, any other formcan be used, if appropriate.

[0035] Referring to FIG. 3, cement 54 is pumped from the surface downthe inside of the drill pipe 28. The cement used is a low densityfibre-reinforced slurry, containing metal fibres to provide a strongerstructure when set. The quantity of slurry pumped from the surface issufficient to fill the annulus 22 and to extend partly into the casedzone 12. Once a sufficient amount of slurry 54 has been pumped into thedrill string 28, a dart 52 is released and pumped down the drill pipe 28with a displacing fluid 56. The cement 54 exits the float shoe 32 viathe side ports 42 and fills the annulus 22 as shown.

[0036] When the dart 52 reaches the landing sub 29, a pressure bump isdetected at the surface and pumping is stopped. At this point, the floatvalve 40 closes and prevents cement 54 from returning into the liner ordrill pipe 28 (FIG. 4).

[0037] Once pumping has stopped, the setting tool 30 is disconnectedfrom the liner 26 by unscrewing the threaded connectors 34, 36, the webs46 holding the liner 26 against rotation, and the lower end of the drillpipe 28 is withdrawn from the stab in connector 38. The pressure in thedrill pipe 28 is raised to a sufficient level to rupture a disc in thedart 52 and allow fluid communication between the drill pipe 28 and theinside of the liner 26. As the drill pipe 28 is withdrawn from the liner26, displacing fluid 56 is pumped to ensure that the cement 54 is notdrawn into the liner 26 and to displace cement 54 in the cased zone 12above the liner 26 (FIG. 5).

[0038] Once the cement has set (FIG. 6), drilling is commenced using amilling bit 58 of the same diameter as the previous cased zone 12. Thisis used to drill down through the liner 26 and part of the cement sheath54 and to drill out the float shoe 32 at the bottom (FIG. 7).

[0039] Once the liner 26 and float shoe 32 are drilled out (FIG. 8) acement sheath 54 is left on the borehole wall in the zone 18. Thus theproblem zone 18 is stabilised and drilling can recommence withoutfurther problems in that zone.

[0040] While the invention has been described in the context ofcementing a problem zone below a cemented borehole, it will beappreciated that it is not restricted to this application. For example,the operation described above can be repeated immediately below a zonethat has already been stabilised in the same way. Alternatively, theregion above might not be cased or cemented if it is itself stable andsupportive. Thus it is possible that the number of casing run in a wellcan be significantly reduced when compared to wells drilled usingprevious techniques, allowing the borehole diameter to be maintained tothe target depth.

[0041] In an alternative embodiment of the invention, the liner has asubstantially constant diameter. In this case, the diameter of the lineris selected such that the portion extending into the zone above leaves asufficient annulus in the upper zone for proper cement placement, bothin the problem zone and in the area of overlap of the liner in the upperzone. In this case, the amount of cement to be drilled out is greater.

[0042] Also, in the case described above, the upper zone has beencemented using a normal casing shoe at the bottom of the upper casing.In another embodiment of the invention, the installation of the liner ispreceded by cementing the casing in the upper zone using a casing shoethat has oversized inside and outside diameters. In this case, theenlargement can be started in the upper zone such that the cement sheathproduced is anchored in the upper zone as well as in the problem zoneleading to greater stability. Alternatively, or in addition, the casingshoe in the zone above can be extended using drillable tubulars to givea similar effect and benefit.

1 A method of stabilising a zone of interest of a borehole, comprising:(i) forming a region of the borehole having enlarged diameter in thezone of interest when compared to the adjacent zone above; (ii)positioning a liner in the borehole extending across the zone ofinterest and into the adjacent zone above, the liner having a pipeextending therethrough to a lower portion and being connected to acement supply at the surface; (iii) pumping cement from the surfaceinside the pipe so as to exit the liner at the lower portion and flowupwards to fill the annulus formed between the outside of the liner andthe borehole in the zone of interest and to extend into the adjacentzone above; (iv) withdrawing the pipe from the liner; and (v) drillingthrough the cement and liner in the zone of interest after the cementhas set to form a borehole of substantially similar diameter to that ofthe adjacent zone above; characterised in that the portion of the linerextending into the adjacent zone above has a smaller diameter that thatpart of the liner in the zone of interest. 2 A method as claimed inclaim 1, wherein the zone above the zone of interest is an open,well-consolidated and stable zone; or lined with cemented casing orsheath. 3 A method as claimed in claim 1 or 2, wherein the zone above isa zone that has been previously stabilised by use of a drilled liner. 4A method as claimed in any preceding claim, wherein the region ofenlarged diameter is formed using an under-reamer or a bi-centre bit. 5A method as claimed in any preceding claim, wherein a bit pilot hole isprovided at the bottom of the enlarged section. 6 A method as claimed inany preceding claim, wherein centralisers are provided to centralise theliner, both in the zone of interest and in the zone above. 7 A method asclaimed in any preceding claim, wherein the liner is positioned in thezone in a fluid-filled state. 8 A method as claimed in claim 7, whereinas the pipe is withdrawn from the liner, fluid is pumped through thepipe to maintain the liner in its fluid-filled state and to displace naycement in the zone above. 9 A method as claimed in any preceding claim,wherein the pipe in the liner is a drill pipe and the liner is connectedto the drill pipe by means of a setting tool. 10 A method as claimed inclaim 9, wherein the drill pipe connects to the upper part of the linerby means of a threaded connector or any other releasable connector. 11 Amethod as claimed in claim 9 or 10, wherein a dart landing sub can beprovided inside the drill pipe near the bottom thereof. 12 A method asclaimed in any of claims 9-11, wherein the drill pipe is connected to afloat shoe at the bottom of the liner which has side ports forcommunication with the annulus and cross webs to engage in the formationat the bottom of the borehole. 13 A method as claimed in any of claims9-12, wherein when the cement has been placed, the drill pipe isdisconnected from the liner and drilling fluid circulated through thedrill pipe inside the liner to prevent cement from filling the liner andto displace cement above the liner. 14 A method as claimed in anypreceding claim, wherein the cement is a fibre-reinforced cement. 15 Amethod as claimed in claim 14, wherein the fibres are metallic or formedfrom a suitable polymeric material. 16 A method as claimed in anypreceding claim, wherein the zone of interest comprises a zone that hasmechanical stability problems, a formation experiencing shear failure orcreep or plastic flow, an unconsolidated formation, a formation withnarrow pore or fracture gradient margins or narrow kick or loss windows,or a formation susceptible to differential sticking problems due tolarge differences between drilling fluid hydrostatic pressure andformation pressure.